17.1 Physical Defenses
3 min read•june 18, 2024
Your body's first line of defense is a complex system of physical barriers and . From the tough, waterproof skin to the -lined cavities, these barriers work tirelessly to keep pathogens out.
But it's not just about keeping things out. Your body also has clever ways to remove threats, like the tiny in your lungs that sweep away invaders. Plus, your acts as a living shield, competing with harmful microbes for space and resources.
Physical Barriers and Mechanical Defenses
Physical barriers against pathogens
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- Physical barriers prevent pathogens from entering the body
- Skin acts as a physical barrier
- in the epidermis provides a tough, waterproof barrier (calluses, corns)
- secrete , which has antimicrobial properties (fatty acids, glycerides)
- Low pH of skin surface inhibits bacterial growth (pH 4-6)
- line body cavities open to the exterior (nose, mouth, eyes)
- Produce that traps microorganisms (glycoproteins, water)
- Contain antimicrobial enzymes like that break down bacterial cell walls
- Skin acts as a physical barrier
- Mechanical defenses remove pathogens from the body
- Cilia in respiratory tract sweep mucus and trapped particles out of the lungs (bronchi, trachea)
- in the gastrointestinal tract propels food and microbes through the system (esophagus, intestines)
- Flushing action of urine and tears washes away potential pathogens (kidneys, lacrimal glands)
- Coughing and sneezing expel pathogens from the respiratory tract (diaphragm, intercostal muscles)
Microbiome as defensive mechanism
- Microbiome refers to the community of microorganisms living in and on the human body
- Includes bacteria, fungi, and viruses (, )
- Most abundant in the gut, skin, and mucous membranes (colon, epidermis, oral cavity)
- Microbiome competes with pathogens for resources
- Nutrients and space are limited, preventing pathogen colonization (carbohydrates, iron)
- Some members of the microbiome produce antimicrobial substances
- are proteins that inhibit the growth of closely related bacterial strains (, )
- Organic acids and hydrogen peroxide produced by microbiome members create an unfavorable environment for pathogens (lactic acid, acetic acid)
- Microbiome stimulates the development and function of the immune system
- Interactions with commensal bacteria help mature the immune system (, )
- Exposure to diverse microbes reduces the risk of allergies and autoimmune diseases ()
Components of skin and mucous defenses
- components
- , the outermost layer of the epidermis
- Composed of dead, keratinized cells tightly bound together (, lipid matrix)
- Provides a physical and water-resistant barrier (transepidermal water loss)
- in the epidermis
- Specialized dendritic cells that detect and present antigens to the immune system (MHC class II, CD1a)
- , the slightly acidic film on the skin surface
- Formed by , sweat, and lactic acid from resident bacteria (pH 4.5-6.5)
- Low pH inhibits the growth of many pathogens (Staphylococcus aureus, Candida albicans)
- , the outermost layer of the epidermis
- components
- form a physical barrier ()
- Connected by that prevent pathogen invasion (, )
- secrete mucus
- Mucus traps and removes microorganisms (glycoproteins, )
- Contains antimicrobial enzymes and antibodies (, )
- Specialized immune cells in the
- Dendritic cells, macrophages, and lymphocytes detect and respond to pathogens (, )
- antibodies in mucus
- Bind to and neutralize pathogens, preventing their attachment to epithelial cells (dimeric IgA, J chain)
- form a physical barrier ()
Innate immunity and additional defense mechanisms
- provides rapid, non-specific protection against pathogens
- Includes physical barriers, cellular components, and chemical mediators
- are produced by various cells in the body
- Act as natural antibiotics, disrupting bacterial cell membranes (defensins, cathelicidins)
- can form on surfaces, providing protection for microorganisms
- Can be both beneficial (as part of the microbiome) and harmful (in infections)
- is found throughout the body
- Provides localized immune responses in mucous membranes (gut-associated lymphoid tissue, bronchus-associated lymphoid tissue)
Key Terms to Review (62)
Acid Mantle: The acid mantle is a thin, slightly acidic film on the surface of the skin that acts as a protective barrier against environmental stressors and harmful microorganisms. This natural defense mechanism is an important part of the skin's physical defenses.
Antimicrobial peptides: Antimicrobial peptides (AMPs) are small proteins that play a crucial role in the innate immune response by directly killing a wide range of pathogens, including bacteria, fungi, and viruses. They are produced by various cells in the body and are particularly abundant in the skin and eyes.
Antimicrobial Peptides: Antimicrobial peptides are small, naturally occurring molecules that act as a first line of defense against a wide range of pathogens, including bacteria, viruses, and fungi. They are an integral part of the innate immune system and play a crucial role in physical and chemical defenses against microbial infections.
Bacteriocins: Bacteriocins are antimicrobial peptides or proteins produced by bacteria that can inhibit or kill closely related bacterial strains. They play a role in bacterial competition and defense within microbial communities.
Biofilms: Biofilms are complex communities of microorganisms, primarily bacteria, that adhere to surfaces and are encased in a self-produced extracellular matrix. These microbial communities exhibit unique characteristics and behaviors that distinguish them from free-floating, planktonic microbes.
Cell junctions: Cell junctions are specialized structures that connect cells to each other or to the extracellular matrix. They play critical roles in maintaining tissue integrity and facilitating communication between cells.
Cilia: Cilia are small, hair-like projections that extend from the surface of certain cells, primarily those found in the respiratory and reproductive systems. These structures play a crucial role in the physical defenses of the body by facilitating the movement of materials and particles across cell surfaces.
Ciliated epithelial cells: Ciliated epithelial cells are specialized cells with hair-like structures called cilia on their surface. They play a critical role in moving mucus and trapped particles out of the respiratory tract.
Claudins: Claudins are a family of integral membrane proteins that are the main components of tight junctions, which are critical for the formation of epithelial and endothelial cell barriers. They are essential for maintaining the selective permeability of these barriers and regulating the movement of molecules between cells.
Colicins: Colicins are protein toxins produced by certain strains of Escherichia coli bacteria. They act as a defense mechanism, allowing the producing bacteria to inhibit or kill closely related competitor strains, thereby providing a competitive advantage in the environment.
Commensals: Commensals are microorganisms that live in close association with a host, such as the human body, without causing harm or providing any significant benefit to the host. They are part of the normal microbiota and coexist peacefully, occupying specific niches within the host environment.
Corneocytes: Corneocytes are the flattened, anucleated cells that make up the outermost layer of the skin, known as the stratum corneum. These cells are the end-stage product of keratinocyte differentiation and play a crucial role in the skin's physical defense mechanisms.
Cystic fibrosis: Cystic fibrosis (CF) is a genetic disorder that affects the respiratory and digestive systems, caused by mutations in the CFTR gene. It leads to thick, sticky mucus production, which obstructs airways and promotes bacterial infections.
Endothelia: Endothelia are thin layers of cells lining the interior surface of blood vessels and lymphatic vessels, forming a barrier between circulating blood or lymph in the lumen and the rest of the vessel wall. They play a crucial role in controlling vascular relaxation and contraction, blood clotting, immune function, and platelet adhesion.
Endothelial cells: Endothelial cells form the thin layer of cells lining the blood vessels and lymphatic vessels. They play a crucial role in barrier function, inflammation, and immune response.
Epithelial cells: Epithelial cells are a type of cell that line the surfaces of your body, including skin, blood vessels, organs, and cavities. They form a protective barrier and play a crucial role in innate nonspecific host defenses.
Epithelium: Epithelium is a type of tissue that forms the outer layer of the body and lines the internal organs and cavities. It acts as a protective barrier, facilitating important functions such as absorption, secretion, and sensory perception.
Fecal transplantation: Fecal transplantation is a medical procedure where fecal matter from a healthy donor is transferred to the gastrointestinal tract of a recipient. It is primarily used to treat Clostridioides difficile infections and other gut microbiota imbalances.
Goblet cells: Goblet cells are specialized epithelial cells that secrete mucus to protect mucous membranes. They play a critical role in trapping pathogens and particles, preventing them from entering the body.
Goblet Cells: Goblet cells are specialized epithelial cells found in the mucous membranes of the respiratory and gastrointestinal tracts. They are responsible for the production and secretion of mucus, a protective and lubricating substance that helps trap and expel foreign particles, pathogens, and irritants from the body.
Hygiene Hypothesis: The hygiene hypothesis suggests that a lack of early childhood exposure to infectious agents, symbiotic microorganisms, and parasites increases susceptibility to allergic and autoimmune diseases by preventing the normal development of the immune system. It proposes that our modern, overly hygienic environments deprive the immune system of necessary stimuli, leading to dysregulation and an increased risk of immune-related disorders.
Innate Immunity: Innate immunity refers to the body's natural, non-specific defense mechanisms that provide immediate protection against pathogens. It is the first line of defense against infectious agents and plays a crucial role in maintaining overall immune function.
Keratin: Keratin is a fibrous structural protein that is the primary component of the outer layer of human skin, hair, and nails. It provides strength, flexibility, and protection to these tissues.
Lamina Propria: The lamina propria is a layer of loose connective tissue that underlies the epithelium in various organs and structures, including the gastrointestinal tract, respiratory system, and urinary system. It serves as a supportive framework and contains a variety of cells and structures that contribute to the organ's function and defense.
Langerhans Cells: Langerhans cells are a type of dendritic cell found in the epidermis, the outermost layer of the skin. They play a crucial role in the skin's immune defense system and are considered part of the physical defenses against pathogens.
Lysozyme: Lysozyme is an enzyme that breaks down the cell walls of bacteria, particularly Gram-positive bacteria, by cleaving the bonds in peptidoglycan. It is a crucial component of the innate immune system and is found in various body fluids such as tears, saliva, and mucus.
Lysozyme: Lysozyme is an enzyme found in various body fluids, such as tears, saliva, and mucus, that acts as a natural antimicrobial agent by breaking down the cell walls of certain bacteria. It is an important component of the body's innate immune system, providing a physical and chemical defense against pathogens.
M Cells: M cells, also known as microfold cells, are specialized epithelial cells found in the mucosa-associated lymphoid tissues, such as the Peyer's patches of the small intestine. These cells play a crucial role in the body's physical defenses by facilitating the transport of antigens and microorganisms from the gut lumen to the underlying immune cells.
Mechanical defenses: Mechanical defenses are physical barriers and processes that prevent the entry or facilitate the removal of pathogens from the body. These include structures like skin and mucous membranes, as well as actions like coughing and sneezing.
Microbiome: The microbiome refers to the diverse community of microorganisms, including bacteria, archaea, viruses, and fungi, that reside in a particular environment, such as the human body. These microorganisms form complex, symbiotic relationships with their host, playing crucial roles in various aspects of health and disease.
Mucociliary escalator: The mucociliary escalator is a defense mechanism in the respiratory tract where mucus traps inhaled particles and cilia move them upwards towards the throat to be expelled or swallowed. This process helps keep the respiratory tract clear of pathogens and debris.
Mucosal-Associated Lymphoid Tissue (MALT): Mucosal-associated lymphoid tissue (MALT) is a specialized collection of lymphoid tissue that is found in the mucous membranes of the body, such as the gastrointestinal tract, respiratory tract, and genitourinary tract. MALT plays a crucial role in the body's physical defenses against pathogens that attempt to enter through these mucosal surfaces.
Mucous membrane: Mucous membranes are moist linings found in various body cavities and passages that open to the external environment. They play a crucial role in trapping pathogens and particulates, thus serving as a first line of defense.
Mucous Membranes: Mucous membranes are moist, protective linings that cover the body's internal surfaces, such as the nose, mouth, throat, lungs, and gastrointestinal tract. They play a crucial role in the body's physical defenses against pathogens and other harmful substances.
Mucus: Mucus is a viscous secretion produced by mucous membranes that serves as a protective barrier and trap for pathogens. It plays a critical role in the immune responses of the respiratory, digestive, and other systems.
Mucus: Mucus is a viscous, slippery secretion produced by specialized cells in the body, primarily in the respiratory, digestive, and urinary tracts. It serves as a critical physical defense mechanism, trapping and removing harmful particles, pathogens, and irritants from the body.
Nisin: Nisin is a polypeptide antimicrobial compound produced by certain strains of the Gram-positive bacterium Lactococcus lactis. It is classified as a bacteriocin and is primarily used as a food preservative due to its ability to inhibit the growth of many Gram-positive bacteria, including Listeria and Clostridium species.
Occludins: Occludins are transmembrane proteins that play a crucial role in the formation and maintenance of tight junctions between epithelial cells, which are important physical defenses against the entry of pathogens and other harmful substances into the body.
Peristalsis: Peristalsis is the rhythmic contraction and relaxation of muscles in the gastrointestinal tract that moves food through the digestive system. It also helps expel pathogens and debris from the body.
Peristalsis: Peristalsis is the rhythmic, wave-like muscle contractions that move food through the digestive tract. It is a crucial mechanism that propels the contents of the digestive system from the esophagus to the anus, ensuring efficient digestion and elimination.
Peyer's Patches: Peyer's patches are organized lymphoid follicles found in the mucosa of the small intestine, particularly the ileum. They play a crucial role in the body's immune defenses and the maintenance of the gut microbiome.
Probiotics: Probiotics are live microorganisms, typically bacteria, that are beneficial to human health, especially the digestive system. They are often referred to as the 'good' bacteria that can help maintain a healthy gut microbiome, support immune function, and promote overall wellbeing.
Resident microbiota: Resident microbiota are the microorganisms that consistently inhabit various sites of a healthy human body, forming a stable and symbiotic relationship. They play a crucial role in protecting against pathogens and maintaining homeostasis.
Rose gardener’s disease: Rose gardener’s disease, also known as sporotrichosis, is a fungal infection caused by Sporothrix schenckii. It typically enters the body through minor cuts or abrasions and affects the skin, lymphatic system, and occasionally the lungs.
Sebaceous glands: Sebaceous glands are small oil-producing glands present in the skin. They secrete sebum, which lubricates and waterproofs the skin and hair.
Sebaceous Glands: Sebaceous glands are specialized exocrine glands found in the skin that produce an oily substance called sebum. These glands play a crucial role in the skin's physical defenses and the normal microbiota of the skin and eyes.
Sebum: Sebum is an oily substance produced by sebaceous glands in the skin. It plays a role in maintaining skin hydration and providing a barrier against microbial invasion.
Sebum: Sebum is an oily, waxy substance produced by the sebaceous glands in the skin. It plays a crucial role in the physical defenses of the skin and the normal microbiota of the skin and eyes.
Secretory IgA: Secretory IgA (sIgA) is an immunoglobulin found predominantly in mucosal areas, such as the gut, respiratory tract, and urogenital tract. It plays a crucial role in immune function by preventing colonization by pathogens.
Secretory IgA: Secretory IgA (sIgA) is a specialized antibody that plays a crucial role in the body's mucosal immune defense system. It is the predominant immunoglobulin found in external secretions, such as saliva, tears, and respiratory and intestinal fluids, where it helps protect against pathogens and maintain a healthy microbiome.
Skin barrier: The skin barrier is the outermost layer of the skin that acts as a physical and chemical defense against pathogens. It prevents harmful microorganisms from entering the body and helps maintain overall skin health.
Sporothrix schenkii: Sporothrix schenkii is a dimorphic fungus responsible for sporotrichosis, an infection typically affecting the skin. It is commonly found in soil, plants, and decaying vegetation.
Sporotrichosis: Sporotrichosis is a chronic fungal infection caused by Sporothrix schenckii, typically entering the skin through small cuts or abrasions. It often manifests as nodular lesions that can ulcerate and spread along lymphatic pathways.
Stratum corneum: The stratum corneum is the outermost layer of the epidermis, composed mainly of dead keratinized cells. It acts as a barrier to protect underlying tissues from infection, dehydration, and physical harm.
Stratum Corneum: The stratum corneum is the outermost layer of the epidermis, the skin's protective barrier. It is a crucial component of the physical defenses of the body and plays a key role in the anatomy and normal microbiota of the skin and eyes.
TH17 cells: TH17 cells are a subset of CD4+ T helper cells that produce the cytokine IL-17 and play a crucial role in defending against extracellular pathogens and contributing to inflammatory responses.
Th17 Cells: Th17 cells are a subset of T helper cells that play a crucial role in the body's immune response, particularly in the context of autoimmune diseases and inflammatory conditions. These cells are characterized by their production of the cytokine interleukin-17 (IL-17), which helps coordinate the recruitment and activation of other immune cells to sites of inflammation.
Tight Junctions: Tight junctions are specialized cell-to-cell adhesion complexes that form continuous seals between adjacent epithelial cells. They act as physical barriers, regulating the movement of molecules and ions through the paracellular space, and are crucial in maintaining the integrity of epithelial tissue layers, such as those found in the skin, gastrointestinal tract, and blood-brain barrier.
Toll-like receptors: Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system by recognizing molecules frequently found on pathogens. They help initiate immune responses and bridge innate and adaptive immunity.
Toll-like Receptors: Toll-like receptors (TLRs) are a class of proteins that play a crucial role in the innate immune system's ability to recognize and respond to pathogenic microorganisms. They act as pattern recognition receptors, detecting conserved molecular structures found in various microbes, triggering an immediate immune response.
Trefoil Factors: Trefoil factors are a family of small, secreted proteins that play a crucial role in maintaining the integrity and repair of mucosal surfaces, particularly in the gastrointestinal tract. These factors are known for their ability to promote cell migration, proliferation, and restitution, which are essential for the protection and regeneration of epithelial tissues.
Vagina: The vagina is a muscular tube that connects the external genitals to the cervix of the uterus. It plays a key role in the female reproductive system and serves as a pathway for menstrual flow, sexual intercourse, and childbirth.